Electroencephalogram (EEG) and electrocardiogram (ECG or EKG) sensors measure the time-varying magnitude of electric fields emanating from the brain and heart, respectively, as a result of cellular activity within the organ. Currently available sensors for measurement of these electrical potentials require direct electrical contact with the skin, which can be achieved by using conductive gel between the sensor and the skin or by abrading the skin. While the gel satisfies the aim of making a good contact, there are several potential drawbacks. First, it can take up to an hour to apply the gel into EEG caps that use 256 sensors. In addition, the gel can diffuse through the hair to create shorts between sensors and can dry out over time, making long term recording very difficult. ECG sensors are often attached to the skin via an adhesive that requires that the attachment area be free of hair, i.e., shaved, and further that the skin area be lightly abraded to produce good contact. Removal of the sensors upon completion of the test is at best unpleasant and usually fairly painful.
There have been many attempts to use sensors that do not require gel, but still rely on dry contact with the skin. Generally, these approaches are limited to body areas with no hair. For example, the ICAP™ Release Meter System, described in U.S. Patent Publ. No. 2007/0048707, is a personal consumer product available from ICAP Technologies for stress management which holds an electrode in place against the user's forehead by way of an elastic headband. A hybrid approach, described in U.S. Pat. No. 6,510,333 of Licata, et al., avoids the need for direct application of gel while still relying on its conductive properties by using soft elastomeric bristles filled with conductive liquid or gels. A disadvantage is that the bristle pads can be relatively expensive to manufacture.
Early, non-contact biopotential sensors have had some success. Prance and co-workers have used low input-bias current amplifiers that yield low-noise operation at low frequencies. (See R. J. Prance, A. Debray, T. D. Clark, H. Prance, M. Nock, C. J. Harland, and A. J. Clippingdale, “An ultra-low-noise electrical-potential probe for human-body scanning”, Measurement Science and Technology, vol. 11, pgs. 291-297, 2000; and C. J. Harland, T. D. Clark and R. J. Prance, “Electric potential probes—new directions in the remote sensing of the human body”, Measurement Science and Technology, vol. 13, pgs. 163-169, 2002.) A drawback of such capacitively coupled electrical sensors is that parasitic charge builds up due to sensor drift and input bias offset currents. The conventional means for counteracting this drift involves including a conductive path to signal ground with a shunting resistor. The problem with such a scheme is that the high-valued resistor that is used contributes excessive amounts of thermal noise, contaminating the signal. U.S. Pat. No. 7,088,175 of Krupka describes a feedback circuit that continuously stabilizes the voltage at the input node of the amplifier. However, such circuits can also introduce noise and have relatively high power requirements.
Accordingly, what is needed is a gel-free non-contact sensor that avoids the need for contact with the skin altogether, is not limited to body areas with no hair, and further avoids the drift and noise problems of the prior art non-contact sensors.